The present invention relates to a back frame for a seat, particularly for a power vehicle seat.
Back frames for seats have a transverse member and side members which are connected with the transverse member and provided at their free end with connecting ears for connecting with hinge parts associated with a seat pad frame. The side members have a substantially U-shaped contour whose side web narrows from the region of its connecting ear to the connecting point with the transverse member. In addition to the side web, the leg rearwardly connected to the side web of each side member also narrows from the connecting ear to the connecting point on the transverse member, and the narrowing leg in the region of its connecting ear forms an asymetrical profile cross section and is longer as compared with the other leg. In the known back frame both side members connected at their upper end by the transverse member have the greatest width at the respective side web in the region of the connecting ears, and the width gradually decreases upwardly toward the transverse member until it reaches a minimal value at the connecting point with the transverse member. This shape of the side members, is selected in dependence upon static requirements and starting from the connecting point of the slide members on the transverse member to the lower connecting ear of each side member corresponds to the increasing course of the bending moment.
It is further known to design the transverse member which connects the side members at the upper end, as a lightweight construction elastically. For increasing stability, the U-shaped profile cross section of the side web is formed with non-uniform legs. With a rigid transverse member, such a back frame is characterized by a force-path diagram which has first strong force increase over small deflection path portion of the side member, analogously to the stress-strain diagram, and shortly before attaining the basic maximal load of the back frame causes, in the event of small force increase, great deflection path. Small deflection paths mean short-time and thereby respectively high action of the accidental acceleration forces upon a person which uses the seat. For a seat user which because of the weight of its body causes a loading which is close to the loading force basic for the back frame, the initial steep curve course of the path-time-diagram is not important inasmuch as in the event of the accident the back frame passes through the entire deflection path so that the acceleration or deceleration forces acting upon the seat user do not exceed the permissable value. However, for a seat user who because of his small weight applies a considerably lower loading upon the back frame in the event of an accident, the possible deflection of the back frame is not utilized, so that the acceleration or deceleration forces act during a considerably shorter time on the lighter seat user. Thereby the permissable value is exceeded and this can lead to death of the lighter seat user.
For providing zones of identical deformation in the side members and thereby forming the deflection path of the side members proportional to the mass of the seated person, it was proposed in the known back frames, in addition to the side web, to narrow in each side member also the rearwardly connected leg in direction to the transverse member and to make this leg longer relative to the other leg, so as to form an asymetrical profile cross section. Thereby turning of the side member in the event of the accidental loading takes place, so that a relatively great deflection path at the transverse member of the back frame is obtained, resulting in a substantially flat curve course in the force-path diagram. This means that in the event of an accident any person, regardless of its weight up to the weight of a maximum acceptable value, is exposed to identical acceleration or decelaration forces. Because of the asymetrical course of the profile cross section over the entire length of the side member the frame is stabilized after a certain deflection path, and with the increasing turning of the side member, a point is attained in which the stability of each side member reaches its maximum value. In certain cases in which either the maximum value is somewhat exceeded, or the accident forces are superposed by side forces, the bending stability of the side member profile designed in accordance to the above mentioned criteria is not reached, whereby the bending back can lead to considerable injury to the seat user. A material reinforcement of the profile walls can reduce the above mentioned desirable turning of the side members in the event of accidental loading; however, the above mentioned advantages will be lost.